A lack of synergy? An unusual actinide-ligand bonding mode

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A lack of synergy? An unusual actinide-ligand
bonding mode
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What should I talk about?

Anything you like, as long as you are
enthusiastic
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Outline of presentation

• Part 1 A very brief introduction to actinide
  chemistry
• The f elements by N Kaltsoyannis and P Scott,
  Oxford University Press (1999)
• The Chemistry of the Actinide and Transactinide
  Elements, 3rd Edition, L. R. Morss, N. Edelstein,
  and J. Fuger (eds), Springer (2006)
• Part 2 Unusual metal-ligand bonding modes in
  molecular uranium complexes

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Just checking..

H He He
Li Be B B C C N N O O F F Ne Ne
Na Mg Al Al Si Si P P S S Cl Cl Ar Ar
K Ca Sc Sc Ti Ti V V Cr Cr Mn Mn Fe Fe Co Co Ni Ni Cu Cu Zn Zn Ga Ga Ge Ge As As Se Se Br Br Kr Kr
Rb Sr Y Y Zr Zr Nb Nb Mo Mo Tc Tc Ru Ru Rh Rh Pd Pd Ag Ag Cd Cd In In Sn Sn Sb Sb Te Te I I Xe Xe
Cs Ba La La Hf Hf Ta Ta W W Re Re Os Os Ir Ir Pt Pt Au Au Hg Hg Tl Tl Pb Pb Bi Bi Po Po At At Rn Rn
Fr Ra Ac Ac Rf Rf Db Db Sg Sg Bh Bh Hs Hs Mt Mt

Ce Ce Pr Pr Nd Nd Pm Pm Sm Sm Eu Eu Gd Gd Tb Tb Dy Dy Ho Ho Er Er Tm Tm Yb Yb Lu Lu  
Th Th Pa Pa U U Np Np Pu Pu Am Am Cm Cm Bk Bk Cf Cf Es Es Fm Fm Md Md No No Lr Lr  
Element 89
Element 90
Element 103
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The ground electronic configurations of the
actinides

Element Electronic configuration Thorium
Rn6d27s2 Protactinium Rn5f26d17s2 Uranium
Rn5f36d17s2 Neptunium Rn5f46d17s2 Plutoniu
m Rn5f67s2 Americium Rn5f77s2 Curium
Rn5f76d17s2 Berkelium Rn5f97s2 Californiu
m Rn5f107s2 Einsteinium Rn5f117s2 Fermium
Rn5f127s2 Mendelevium Rn5f137s2 Nobelium
Rn5f147s2 Lawrencium Rn5f146d17s2
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The shapes of the seven 5f orbitals (cubic set).
5fy3,
5fx3, 5fz3

5fx(z2-y2), 5fy(z2-x2), 5fz(x2-y2)
5fxyz
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The oxidation states adopted by the actinide
elements in their compounds
The most stable oxidation state in aqueous
solution is represented by the black circles.
Open circles indicate other oxidation states
adopted and squares indicate that the oxidation
state is found only in solids.
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Radial distribution functions of selected atomic
orbitals of U6 (Enrique Batista, B3LYP,
all-electron, 2nd order DK)

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The particular challenges posed to quantum
chemistry by the actinides
1 Lots of electrons. 2 Heavy elements ?
relativistic effects are important (scalar -
modification of atomic orbital energies and
spin-orbit). 3 Large number of valence atomic
orbitals of similar radial distribution and
energy (5f, 6p, 6d, 7s, 7p) ? actinide complexes
are frequently open-shell, with many
closely-spaced electronic states. The correct
description of electron correlation effects is
extremely important (and difficult) in these
cases.
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Part 2 Unusual metal-ligand bonding modes in
molecular uranium complexes The classic
Dewar-Chatt-Duncanson view of synergic bonding

Qualitative MO scheme for CO
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Qualitative MO scheme for octahedral ML6 with p
acceptor ligands (e.g. CO)
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Are there CO complexes of the actinides?

(L)U2(µh1,h1-CO) Meyer et al. JACS 127
(2005) 11242
The hard, oxophilic f elements typically have a
low binding affinity for the soft p bonding CO
ligand, and carbonyl complexes do not readily
form
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f orbital to carbonyl 2p backbonding the
electronic structures of (C5H5)3U(CO) and
(C5H5)3U(OC)

B.E. Bursten and R.J. Strittmatter, JACS 109
(1987) 6606.
Two major interactions of (C5H5)3U(CO) are
discussed. The CO 3s lone pair interacts
primarily with the empty U 6d orbitals to form
the U-CO s bond, and extensive U 5f ? CO 2p
backbonding is observed
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P. Roussel and P. Scott, JACS 120 (1998) 1070.
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? Back bonding without s bonding
N.Kaltsoyannis and P. Scott, Chem. Commun. (1998)
1665.
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What is the oxidation state of the uranium atoms
in (C5Me5)2U2(h-µ6µ6-C6H6)?

Realistic possibilities include (a) U(II) and
neutral benzene (b) U(III) and (benzene)2- (most
likely from experiment) and (c) U(IV) and
(benzene)4-
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How well does calculation reproduce the
experimental geometry?

Interatomic distance/Å Exp. Calc.
U-U 4.396 4.406
U1-Cp (av) 2.840 2.860
U2-Cp (av) 2.830 2.840
C-C (benzene, complex) 1.440 1.440
C-C (benzene, free) 1.390 1.394
U1-C (benzene, av) 2.621 2.634
U1-C (benzene, max) 2.733 2.719
U1-C (benzene, min) 2.547 2.591
U2-C (benzene, av) 2.628 2.627
U2-C (benzene, max) 2.730 2.674
U2-C (benzene, min) 2.538 2.532
So why is the benzene ring so non-planar?
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Hückel energies of the carbocyclic ring p
orbitals

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Calculation suggests (a) each uranium gives up
two electrons to the cp ligands (b) each
uranium has two 5f-based electrons (c) four
electrons (two per uranium) are used to form a
uranium/arene d bond

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• The localisation properties of the four
  uranium/arene d bonding electrons determine the
  formal oxidation state of the metal centres.
• Population analysis indicates that these
  electrons have an approximately equal
  contribution from both metal and arene, and hence
  the oxidation state of the uranium atoms is best
  described as 3.
• The benzene ring is not neutral. Rather, it
  carries a charge close to -2, as there is
  transfer of uranium 5f electron density into the
  benzene e2u C-C p molecular orbitals. The
  benzene ring is thus no longer Hückel aromatic,
  and is significantly non-planar as a result.

W.J. Evans, S.A. Kozimor, J. W. Ziller and N.
Kaltsoyannis, JACS 126 (2004) 14533.
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Arene-bridged diuranium complexes inverted
sandwiches supported by d backbonding
P.L. Diaconescu, P.L. Arnold, T.A. Baker, D.J.
Mindiola and C.C. Cummins, JACS 122 (2000) 6108.